1. Field of the Invention
This invention is directed to a system for modifying the operation of a heat pump system to increase the temperature of air initially delivered thereby. In particular, this invention is directed to a system for increasing the temperature of air initially delivered by an existing heat pump system in a heating mode by adding supplemental heat responsive to operation of controller and duct thermostat interfaced with the heat pump system. More in particular, the controller of the present invention includes a thermostat for sensing outdoor ambient temperature for output of a control signal to energize supplemental heating units of the heat pump system when the ambient air temperature is below a predetermined value. Still further, the system of the present invention includes a duct thermostat which monitors the temperature of the main supply air duct for cutting off the addition of supplemental heat when the air duct temperature reaches a predetermined value.
2. Prior Art
Over the years, heat pump systems have gained great acceptance due to their high efficiency operation in moderate climates. However, as the outdoor ambient temperature approaches freezing, the efficiency of heat pump systems is reduced. Effects such as lubricant migration and reduced heat transfer that occur when the outdoor ambient temperature is low cause the compressor to operate for longer periods of time in order to meet the heating demands being called for by the indoor thermostat. Such heat pump systems add supplemental heat, usually from a source such as one or more stages of electric resistance heating units, when the differential between the indoor ambient temperature and the temperature called for by the thermostat exceeds a predetermined value, usually 2.5.degree. F. During the time period when the compressor is first energized and until the temperature differential reaches the point at which the supplemental heating units are energized, relatively cool air is displaced by the air handler of the heat pump system. This circulation of relatively cool air has brought about the majority of consumer complaints with respect to heat pump operation.
In an attempt to remedy that problem, some systems have been sold which add supplemental heat to raise the temperature of the air exiting the supply air ducts. Such systems, however, add supplemental heat whenever the heating mode is being utilized, irrespective of the outdoor ambient temperature. Thus, such systems are inefficient in that they add heat when such addition is not necessary, thereby significantly increasing the cost of operation of the heat pump system. Whereas in the instant invention, the addition of supplemental heat is only initiated when the outdoor ambient temperature falls below a predetermined value. Further, a lockout mechanism is provided for preventing the addition of such supplemental heat when the heat pump system is not in a heating mode.